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E03-104: Probing the Limits of the Standard Model of Nuclear Physics with the 4He(e,e'p)3H Reaction

Do nucleons change their internal structure inside a nucleus?

For decades, nuclear physicists have wondered whether nucleons change their properties when bound inside a nucleus. This possibility was first thrust into prominence with the discovery of the "nuclear EMC effect" some twenty years ago. There electron scattering from quarks inside a nucleus was discovered to differ significantly from electron scattering from quarks in a free nucleon.

JLab and Mainz experimental data, which compared the properties of protons bound inside the Helium-4 nucleus with those of a free proton (a hydrogen nucleus), provided hints that it may be more economical to describe nuclei in terms of nucleons which differ in size from free nucleons. An alternate explanation in terms of a specific rescattering process between the protons and neutrons of Helium-4 has also been suggested.

The E03-104 experiment provides a more precise measurement that can differentiate between these two explanations. Hence, E03-104 provides one of the best opportunities for uncovering how the proton's properties are modified by the nuclear medium. This could then be confirmed to be the second indication that a nucleus is not merely a set of bound nucleons.